StackContainer

template<typename T, size_t stack_capacity>

class StackAllocator : public std::allocator<T> {

 public:

  typedef typename std::allocator<T>::pointer pointer;

  typedef typename std::allocator<T>::size_type size_type;

 

  // Backing store for the allocator. The container owner is responsible for

  // maintaining this for as long as any containers using this allocator are

  // live.

  struct Source {

    Source() : used_stack_buffer_(false) {

    }

 

    // Casts the buffer in its right type.

    T* stack_buffer() { return reinterpret_cast<T*>(stack_buffer_); }

    const T* stack_buffer() const {

      return reinterpret_cast<const T*>(stack_buffer_);

    }

 

    //

    // IMPORTANT: Take care to ensure that stack_buffer_ is aligned

    // since it is used to mimic an array of T.

    // Be careful while declaring any unaligned types (like bool)

    // before stack_buffer_.

    //

 

    // The buffer itself. It is not of type T because we don't want the

    // constructors and destructors to be automatically called. Define a POD

    // buffer of the right size instead.

    char stack_buffer_[sizeof(T[stack_capacity])];

 

    // Set when the stack buffer is used for an allocation. We do not track

    // how much of the buffer is used, only that somebody is using it.

    bool used_stack_buffer_;

  };

 

  // Used by containers when they want to refer to an allocator of type U.

  template<typename U>

  struct rebind {

    typedef StackAllocator<U, stack_capacity> other;

  };

 

  // For the straight up copy c-tor, we can share storage.

  StackAllocator(const StackAllocator<T, stack_capacity>& rhs)

      : source_(rhs.source_) {

  }

 

  // ISO C++ requires the following constructor to be defined,

  // and std::vector in VC++2008SP1 Release fails with an error

  // in the class _Container_base_aux_alloc_real (from <xutility>)

  // if the constructor does not exist.

  // For this constructor, we cannot share storage; there's

  // no guarantee that the Source buffer of Ts is large enough

  // for Us.

  // TODO: If we were fancy pants, perhaps we could share storage

  // iff sizeof(T) == sizeof(U).

  template<typename U, size_t other_capacity>

  StackAllocator(const StackAllocator<U, other_capacity>& other)

      : source_(NULL) {

  }

 

  explicit StackAllocator(Source* source) : source_(source) {

  }

 

  // Actually do the allocation. Use the stack buffer if nobody has used it yet

  // and the size requested fits. Otherwise, fall through to the standard

  // allocator.

  pointer allocate(size_type n, void* hint = 0) {

    if (source_ != NULL && !source_->used_stack_buffer_

        && n <= stack_capacity) {

      source_->used_stack_buffer_ = true;

      return source_->stack_buffer();

    } else {

      return std::allocator<T>::allocate(n, hint);

    }

  }

 

  // Free: when trying to free the stack buffer, just mark it as free. For

  // non-stack-buffer pointers, just fall though to the standard allocator.

  void deallocate(pointer p, size_type n) {

    if (source_ != NULL && p == source_->stack_buffer())

      source_->used_stack_buffer_ = false;

    else

      std::allocator<T>::deallocate(p, n);

  }

 

 private:

  Source* source_;

};

 

// A wrapper around STL containers that maintains a stack-sized buffer that the

// initial capacity of the vector is based on. Growing the container beyond the

// stack capacity will transparently overflow onto the heap. The container must

// support reserve().

//

// WATCH OUT: the ContainerType MUST use the proper StackAllocator for this

// type. This object is really intended to be used only internally. You'll want

// to use the wrappers below for different types.

template<typename TContainerType, int stack_capacity>

class StackContainer {

 public:

  typedef TContainerType ContainerType;

  typedef typename ContainerType::value_type ContainedType;

  typedef StackAllocator<ContainedType, stack_capacity> Allocator;

 

  // Allocator must be constructed before the container!

  StackContainer() : allocator_(&stack_data_), container_(allocator_) {

    // Make the container use the stack allocation by reserving our buffer size

    // before doing anything else.

    container_.reserve(stack_capacity);

  }

 

  // Getters for the actual container.

  //

  // Danger: any copies of this made using the copy constructor must have

  // shorter lifetimes than the source. The copy will share the same allocator

  // and therefore the same stack buffer as the original. Use std::copy to

  // copy into a "real" container for longer-lived objects.

  ContainerType& container() { return container_; }

  const ContainerType& container() const { return container_; }

 

  // Support operator-> to get to the container. This allows nicer syntax like:

  //   StackContainer<...> foo;

  //   std::sort(foo->begin(), foo->end());

  ContainerType* operator->() { return &container_; }

  const ContainerType* operator->() const { return &container_; }

 

#ifdef UNIT_TEST

  // Retrieves the stack source so that that unit tests can verify that the

  // buffer is being used properly.

  const typename Allocator::Source& stack_data() const {

    return stack_data_;

  }

#endif

 

 protected:

  typename Allocator::Source stack_data_;

  Allocator allocator_;

  ContainerType container_;

 

  DISALLOW_EVIL_CONSTRUCTORS(StackContainer);

};

 

// StackString

template<size_t stack_capacity>

class StackString : public StackContainer<

    std::basic_string<char,

                      std::char_traits<char>,

                      StackAllocator<char, stack_capacity> >,

    stack_capacity> {

 public:

  StackString() : StackContainer<

      std::basic_string<char,

                        std::char_traits<char>,

                        StackAllocator<char, stack_capacity> >,

      stack_capacity>() {

  }

 

 private:

  DISALLOW_EVIL_CONSTRUCTORS(StackString);

};

 

// StackWString

template<size_t stack_capacity>

class StackWString : public StackContainer<

    std::basic_string<wchar_t,

                      std::char_traits<wchar_t>,

                      StackAllocator<wchar_t, stack_capacity> >,

    stack_capacity> {

 public:

  StackWString() : StackContainer<

      std::basic_string<wchar_t,

                        std::char_traits<wchar_t>,

                        StackAllocator<wchar_t, stack_capacity> >,

      stack_capacity>() {

  }

 

 private:

  DISALLOW_EVIL_CONSTRUCTORS(StackWString);

};

 

// StackVector

//

// Example:

//   StackVector<int, 16> foo;

//   foo->push_back(22);  // we have overloaded operator->

//   foo[0] = 10;         // as well as operator[]

template<typename T, size_t stack_capacity>

class StackVector : public StackContainer<

    std::vector<T, StackAllocator<T, stack_capacity> >,

    stack_capacity> {

 public:

  StackVector() : StackContainer<

      std::vector<T, StackAllocator<T, stack_capacity> >,

      stack_capacity>() {

  }

 

  // We need to put this in STL containers sometimes, which requires a copy

  // constructor. We can't call the regular copy constructor because that will

  // take the stack buffer from the original. Here, we create an empty object

  // and make a stack buffer of its own.

  StackVector(const StackVector<T, stack_capacity>& other)

      : StackContainer<

            std::vector<T, StackAllocator<T, stack_capacity> >,

            stack_capacity>() {

    this->container().assign(other->begin(), other->end());

  }

 

  StackVector<T, stack_capacity>& operator=(

      const StackVector<T, stack_capacity>& other) {

    this->container().assign(other->begin(), other->end());

    return *this;

  }

 

  // Vectors are commonly indexed, which isn't very convenient even with

  // operator-> (using "->at()" does exception stuff we don't want).

  T& operator[](size_t i) { return this->container().operator[](i); }

  const T& operator[](size_t i) const {

    return this->container().operator[](i);

  }

};